1. Field of the Invention
This invention relates to a slurry providing system, and more particularly to a slurry providing system that can either act as a buffer tank of slurry to continuously provide slurry to a chemical mechanical polishing (CMP) machine, or as an independent backup tank.
2. Description of Related Art
As the integration of semiconductor devices increases, the size of the metal oxide semiconductor (MOS) transistor is reduced accordingly. This, in turn, increases the requirements for interconnects. Therefore, a design including at least two metal layers is gradually becoming necessary for most integrated circuits. In this kind of design, in order to more easily fabricate devices and more precisely transfer circuit patterns, it is important to planarize the wafer whose surface is usually not level due to the many-layered structures formed on it. Furthermore, planarizing the wafer is the main factor affecting the precision of the alignment. A poor planarization causes imprecise alignment of a mask to the wafer and also induces a higher rate of error in fabrication.
So far, chemical-mechanical polishing (CMP) is the only process capable of globally planarizing a very large scale integration (VLSI) structure or even an ultra large semiconductor integration (ULSI) structure. CMP uses a machine similar to a knife grinder along with a chemical reagent to mechanically grind the uneven profile of the wafer so that it is planarized.
In CMP, the reagent is usually referred to as a slurry. Slurry usually includes a solution mixed with silica in colloidal phase or materials in dispersed phase such as aluminum, KOH or NH.sub.4 OH. The grinding particles are extremely hard and have a diameter of about 0.1-0.2 .mu.m. Basically, these particles are used to polish the wafer surface.
Conventionally, slurry used in CMP is provided from a slurry providing system. One slurry providing system usually provides slurry for several CMP machines as shown in FIG. 1. FIG. 1 is a flow chart schematically illustrating a conventional system for providing slurry to several CMP machines.
When performing a CMP process, slurry is continuously provided from a main slurry providing system 10 to several CMP machines, such as CMP machines 12, 14, 16, and 18, through a transportation route represented by the connecting lines.
For the conventional main slurry providing system, if a short circuit or a breakdown occurs on the main slurry providing system 10, the main slurry providing system 10 cannot continue its normal provision of slurry to the CMP machines 12, 14, 16, 18 to polish wafers (not shown). The fabrication online works are therefore interrupted.
Moreover, if the transportation route (line) for transporting slurry is too long, then pressure used to drive the slurry may become insufficient to properly provide slurry to the CMP machines 12, 14, 16, 18. In this case, the CMP machines 12, 14, 16, 18 do not perform as desired and cause fabrication failure. Furthermore, when the main slurry providing system 10 is in an idle stage due to the interruption, slurry then is standstill and is be out as a crystalline deposit (not shown). In this case, if slurry is reused in CMP, the crystalline deposit will scratch wafers (not shown), which are to be polished, causing yet another kind of fabrication failure.